1. Technical Field
The present invention relates to a polyimide for optical communications and a method for preparing the same. More particularly, the invention relates to a polyimide which is useful as a material for optical waveguiding in optical communications by minimizing light absorption loss in a near infrared light wavelength range, and to a polyimide which has good heat resistance and film processing properties.
2. Related Art
The range of light wavelength for optical communications has been shifted from 800 nm to 1,550 nm belonging to the near infrared light wavelength range. Thus, it is ideal to manufacture an optical communication device using a material which barely absorbs light belonging to the wavelengths of the near infrared light wavelength range.
In general, a polymer is used as an optical substrate such as an optical lens or compact disk. Recently, research into the use of a polymer as a material for an optical waveguide for light transfer in the near infrared light wavelength range has been conducted.
However, a general polymer absorbs light having a wavelength of 1,000.about.1,700 nm, which corresponds to the near infrared wavelength range. Such absorption of near infrared light by the polymer is caused by overtone of harmonics according to stretching and deformation vibrations of the carbon and hydrogen bond (C--H) in allyl, phenyl and other similar functional groups. Thus, using the polymer as a material for an optical waveguide utilizing light in the near infared wavelength range results in a large optical loss. In order to reduce the optical loss, the light absorption wavelength of the polymer should be shifted from the near infrared light wavelength region to a longer or shorter wavelength region. To this end, a method for replacing hydrogen of the carbon and hydrogen (C--H) bond with fluoride (F) or heavy hydrogen (D) has been suggested.
In particular, the method for replacing the hydrogen of the C--H bond with D is not suitable for a material for an optical communications device utilizing light of 1,500 nm because material having method which involves replacing the hydrogen with F can minimize optical loss in light absorption at a wavelength of 1,000.about.1,700 nm.
Also, an optical material for manufacturing an optical device, such as an opto-electronic integrated circuit (OEIC), an opto-electrical mixed wiring board (OEMWB), a hybrid integration device, a plastic optical fiber or a multi-chip module (MCM), requires thermal stability at 250.degree. C. lasting at least for 30 minutes. Because such thermal resistance of the optical material is very important, glass transition temperature, thermal decomposition temperature, thermal expansion coefficient and birefrigency of the optical material should be carefully considered.
Polyimides are widely known as a materials having good thermal resistance. Because the polyimide has a resistance to heat at a high temperature, (e.g., approximately 400.degree. C.), great efforts are being made to utilize polyimide as a material for optical communications.
However, in general, the ordinary polyimide has many C--H bonds within its molecule, and optical loss due to light absorption is large at the near infrared region. To solve this problem, a polyimide whose hydrogen of the C--H bond is partially or completely replaced with fluorine (F) has been reported.
However, when replacing the hydrogen of the C--H bond of the polyimide with F as described above, the refractive index of the polyimide decreases. The content of F in the polyimide is proportional to the decreased level of the refractivity index. Thus, because the polyimide whose hydrogen of the C--H bond is replaced with F (i.e., fluorinated polyimide), has a low reflective index, the range of selection of a material capable of being used for cladding becomes narrow when fluorinated polyimide is used as a material for the core of an optical fiber.
Also, the higher the fluorine content in the polyimide is, the lower the surface tension of the polymer is. Thus, it is difficult to coat a substrate with such a polymer, and a film formed from such polymer shows poor adhesion properties. As a result, film characteristics are deteriorated and the film is very fragile. Thus, in actual practice, it is difficult to use a polyimide as a material for optical waveguiding.